From the prior art it is known, in automatic transmissions, to use form-locking and in particular claw-type shifting elements as coupling elements in place of frictional shifting elements. This has the advantages that production costs are lower and less structural fitting space is needed. However, form-locking shifting elements have the disadvantage that due to their structure, they can only be synchronized against rotational speed differences by elaborate means.
For that reason form-locking shifting elements, in particular claw shifting elements, can only be used in automatic transmissions when they are disengaged in higher gears and engaged in lower gears. Ideally, form-locking shifting elements are always used when they can remain engaged in all the low gears and in the reversing gear, whereby the number of actuations of form-locking shifting elements is restricted to a minimum.
If a form-locking shifting element of an automatic transmission is in the disengaged condition when the internal combustion engine is switched off, it must be brought to the engaged condition when the engine has been started and the hydraulic pressure supply to the transmission has been built up. This means that in hydraulic systems in which the pressure supplied is directly coupled to the rotational speed of the internal combustion engine, simple engagement of the form-locking shifting element is not possible.
To reduce power losses in hydraulic systems of automatic transmissions, in the prior art electrically driven oil pumps are used, which as a rule serve as auxiliary oil pumps. In most cases these oil pumps act in parallel with the oil pump that is driven mechanically by the internal combustion engine, which latter can only ensure an oil supply when the internal combustion engine is running at a certain minimum speed. For example, in DE 103 112 70 A1 a hybrid drive with an oil pump is described, which is powered by an electric motor via a gear system with constant gear ratio.
Furthermore, from DE 10 2005 013 137 A1 by the present applicant a method and a device are known for controlling an oil supply for an automatic transmission and a starting element, such that by means of an oil pump that can be powered mechanically by an internal combustion engine and a second oil pump that can be powered electrically, a sufficient oil supply for the hydraulic control unit of the automatic transmission and/or for the starting element, in particular an oil-cooled friction clutch, can be ensured both when the internal combustion engine is running and when it is off.
From DE 10 2008 001 277 A1 by the present applicant a method is known for operating a drivetrain of a motor vehicle, such that the drivetrain comprises at least a hybrid drive system with an internal combustion engine and an electric motor, a clutch connected between the internal combustion engine and the electric motor, a transmission arranged between the hybrid drive system and a drive output, with which a main hydraulic pump and an auxiliary hydraulic pump are associated, and a transmission-internal or transmission-external starting element.
According to the known method it is provided that when the internal combustion engine is at rest and the clutch connected between the internal combustion engine and the electric motor and the starting element are open, the procedure for electric motor starting is that with the brake pedal actuated, the auxiliary hydraulic pump is first operated at a first power level in such manner that it provides for the transmission a pressure required for holding a defined gear, and then, when a wish to start is recognized, the auxiliary pump is changed to a second power level such that at this second power level it provides the transmission with a pressure required to produce a power torque; only after the lapse of a defined time interval after the auxiliary hydraulic pump is changed from the first to the second power level, is the speed of the electric motor increased in order to provide the desired starting power or the desired starting torque.